1. Field of the Invention
The present invention relates to a vacuum pressure control apparatus for controlling vacuum pressure in a vacuum vessel to a predetermined value, the vacuum vessel being used in semiconductor producing processes, and particularly to a vacuum pressure control apparatus for accurately and promptly controlling vacuum pressure in a vacuum vessel to a predetermined value in a low vacuum region near atmospheric pressure.
2. Description of Related Art
One of vacuum pressure control systems which have conventionally been used in semiconductor producing processes is described in U.S. Patent application No. 08/580,358 filed by applicant of the present invention. This vacuum pressure control system uses a vacuum proportional opening and closing valve which is disposed between a vacuum chamber and a vacuum pump and regulates the quantity of gas flow to be sucked by the vacuum pump to regulate the internal pressure value of the vacuum chamber into a predetermined vacuum pressure. The vacuum proportional opening and closing valve is provided with a poppet valve as a main element. Referring to FIG. 8, the structure of this poppet valve is described.
FIG. 8 shows a part of the vacuum proportional opening and closing valve in a closed state. FIG. 9 shows the same in an open state to provide a medium vacuum region in the vacuum chamber.
A poppet valve 133 has a valve member 133a connected to a piston rod not shown, an annular groove 133b which fixedly holds an O-ring 135, and a connection portion 133c to which a valve element 134 made from stainless steel is assembled. The O-ring 135 will be depressed against the upper surface of a valve seat 136 constructed of a cylindrical inner surface formed in a lower center of a valve body 145 of the vacuum proportional opening and closing valve when a tapered surface 134a formed on the outer periphery of the stainless valve element 134 of the poppet valve 133 is fully inserted in the valve seat 136. Accordingly, the O-ring 135 serves to prevent leakage of fluid through a flow passage defined by the valve seat 136 of the valve body 145 having a port 139 formed below the valve seat 136 and connected to the vacuum chamber. The inclination angle xcex8 of the tapered surface 134a of the stainless valve element 134 is set to 3 degrees in the U.S. Patent application No. 08/580,358. The stainless valve element 134 has also a straight surface 134b above the tapered surface 134a. 
As shown in FIG. 9, when the stainless valve element 134 with the tapered surface 134a is moved along the center line (in a vertical direction in figure) of the valve seat 136, an annular clearance between the tapered surface 134a and the valve seat 136 changes in sectional area. In response to the clearance area, the opening degree of the vacuum proportional opening and closing valve is changed. As shown in FIG. 8, when the poppet valve 133 comes into contact with the upper surface of the valve seat 136, pressing the O-ring 135 against the upper surface of the valve seat 136, the O-ring 135 completely blocks the flow passage to prevent a leakage flow of fluid.
However, the above conventional vacuum pressure control system and control apparatus have the following problems.
Specifically, the clearance area between the tapered surface 134a and the valve seat 136 is changed to regulate the quantity of fluid flow sucked by the vacuum pump. This can relatively facilitate the provision of the flow quantity corresponding to high vacuum pressure near high vacuum or medium vacuum pressure.
The clearance area between the tapered surface 134a and the valve seat 136 is relatively large, however, and it is difficult to regulate the quantity of fluid flow so as to maintain the pressure in the vacuum chamber in a low vacuum region near atmospheric pressure, in particular, extremely close to atmospheric pressure. Such the regulation of the flow quantity to very-low-vacuum pressure is much more difficult when the O-ring made of rubber is used because it is prone to come into intimate contact with the flat upper surface of the valve seat 136. When process gas precipitates on the inside of the valve body 145 and others, the contact degree of the O-ring 135 to the valve seat 136 increases, and the poppet valve 133 will quickly change its moving distance due to the change in the friction coefficient which shifts to dynamical friction from static friction in association with the movement of the poppet valve 133, causing the O-ring 135 to instantaneously separate from the flat upper surface of the valve seat 136. This disables the control to accurately provide a very small clearance between the tapered surface 134a and the valve seat 136.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide a vacuum pressure control apparatus capable of accurately controlling the internal pressure in a vacuum vessel in a low vacuum region near atmospheric pressure.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the purpose of the invention, there is provided a vacuum pressure control apparatus for controlling vacuum pressure in a vacuum vessel from which gas is sucked by means of a vacuum pump, the apparatus including a vacuum proportional opening and closing valve which is disposed between the vacuum pump and the vacuum vessel and changes its opening for controlling the vacuum pressure in the vacuum vessel, the vacuum proportional opening and closing valve including a valve seat having a central flow passage through which the gas is sucked from the vacuum vessel to the vacuum pump, and a valve member that is movable to come into contact with the valve seat and to separate from the same, the valve member being provided with an elastic sealing member on a surface that comes into contact with the valve seat, and the vacuum pressure control apparatus controlling a change in an elastic deformation amount of the elastic sealing member which is in contact with the valve seat to regulate the quantity of gas leakage between the elastic sealing member and the valve seat and to provide a predetermined vacuum pressure in the vacuum vessel.
In the vacuum pressure control apparatus, preferably, a force applied to the elastic sealing member which is in contact with the valve seat through the valve member is controlled to change the elastic deformation amount of the elastic sealing member.
For instance, the vacuum proportional opening and closing valve may include a pilot cylinder for moving the valve member, and the vacuum pressure control apparatus controls a change in air pressure to be supplied to the pilot cylinder to thereby control the force applied to the elastic sealing member which is in contact with the valve seat through the valve member and change the elastic deformation amount of the elastic sealing member.
Next, operation of the vacuum pressure control apparatus configured in the above manner is described below.
The vacuum pump connected to the vacuum vessel makes a constant suction of gas from the vacuum vessel. By changing the opening degree of the vacuum proportional opening and closing valve disposed between the vacuum vessel and the vacuum pump, the quantity of gas flow sucked into the vacuum pump from the vacuum vessel is regulated, thus changing vacuum pressure in the vacuum vessel.
In order to regulate the pressure in the vacuum vessel to high or medium vacuum pressure, the opening degree of the vacuum proportional and opening valve is adjusted in the manner that the valve element is moved by the pilot cylinder along the center line of a flow passage formed in a center of the valve seat, changing a clearance area between the valve seat and the valve element.
In order to regulate the pressure in the vacuum vessel to low vacuum pressure near atmospheric pressure, the vacuum pressure control apparatus operates to change an elastic deforming amount of the elastic sealing member which is in contact with the valve seat and thereby change the quantity of gas leakage through the elastic sealing member.